Flu seasons are always unpredictable, but it can be argued that over the past several years we've seen some unusual flu activity around the world. A few examples of some of the strangeness we've observed includes:
- The most recent (2018-2019) North American flu season started off as a fairly mild H1N1 year, and switched to a more potent H3N2 season in February, featuring a rising clade 3C.3a leading the charge (see see MMWR: Influenza Activity in the United States During the 2018–19 Season).
- The 2017-2018 North American Flu season was the deadliest since the 1968 H3N2 pandemic (see CDC: The Estimated Burden Of Last Year's Flu Season).
- During the summer of 2017, Hong Kong, Taiwan, and parts of Southern China saw an unusually severe, summer H3N2 summer flu season (see Hong Kong's Summer Flu Appears To Have Peaked).
- And as early as early January (early summer in the Southern Hemisphere), Australia began reporting record numbers of `inter-seasonal' flu cases (see April's Australia: An Early Surge In Flu Cases).
Today, the ECDC's Eurosurveillance carries an in-depth look at this unseasonable flu epidemic. As you'll see, while a number of possible reasons are cited, there's no single identifiable factor.Some excerpts follow but due to its length and complexity, there's no way to do it justice here. Highly recommended that you click the link to read it in its entirety.
Surveillance Open Access Intense interseasonal influenza outbreaks, Australia, 2018/19
Ian G Barr1,2, Yi Mo Deng1, Miguel L Grau3, Alvin X Han4,5, Robin Gilmour6, Melissa Irwin7, Peter Markey8, Kevin Freeman9, Geoff Higgins10, Mark Turra10, Naomi Komadina1, Heidi Peck1, Robert Booy11,12, Sebastian Maurer-Stroh4,5,13, Vijaykrishna Dhanasekaran1,3, Sheena Sullivan1,2
Interseasonal influenza outbreaks are not unusual in countries with temperate climates and well-defined influenza seasons. Usually, these are small and diminish before the main influenza season begins. However, the 2018/19 summer-autumn interseasonal influenza period in Australia saw unprecedented large and widespread influenza outbreaks.
Our objective was to determine the extent of the intense 2018/19 interseasonal influenza outbreaks in Australia epidemiologically and examine the genetic, antigenic and structural properties of the viruses responsible for these outbreaks.
This observational study combined the epidemiological and virological surveillance data obtained from the Australian Government Department of Health, the New South Wales Ministry of Health, sentinel outpatient surveillance, public health laboratories and data generated by the World Health Organization Collaborating Centre for Reference and Research on Influenza in Melbourne and the Singapore Agency for Science, Technology and Research.
There was a record number of laboratory-confirmed influenza cases during the interseasonal period November 2018 to May 2019 (n= 85,286; 5 times the previous 3-year average) and also more institutional outbreaks, hospitalisations and deaths, than what is normally seen.
The unusually large interseasonal influenza outbreaks in 2018/19 followed a mild 2018 influenza season and resulted in a very early start to the 2019 influenza season across Australia. The reasons for this unusual event have yet to be fully elucidated but are likely to be a complex mix of climatic, virological and host immunity-related factors. These outbreaks reinforce the need for year-round surveillance of influenza, even in temperate climates with strong seasonality patterns.
In 2018, the Australian influenza season was late and progressed with such minimal activity that it barely registered as a season by several surveillance indicators . This was in stark contrast to the 2017 season, when Australia’s highest levels of influenza activity were recorded .
However, several surveillance indicators suggested that the influenza activity seen in 2018, while low, never really stopped, as it was expected to, at the end of the southern hemisphere spring (November). Instead, Australia experienced an upsurge in influenza cases with a large wet-season outbreak in the tropical north (see Figure 1), while southern Australia saw record numbers of laboratory-confirmed influenza notifications, increased hospitalisations and dozens of influenza-related deaths in late summer and early autumn, resulting in an early start to the 2019 influenza season throughout the country.
Here we summarise the available epidemiological surveillance indicators along with a virological analysis of the influenza viruses collected during these 2018/19 interseasonal influenza outbreaks.(Continue . . . )